* Fix bug in MLS user removal from conversation

The list of removed clients has to be compared with those in the
conversation, not the list of *all* clients of that user.

* Rename getMLSClients to getClientInfo

* Skip client check for deleted clients

The code skipping this check was incorrectly deleted in 49782f5. This
restores it (in a different form) and adds some comments explaining the
logic.